The high heat of combustion of phosphorus to phosphorus pentoxide and ultimately to produce phosphoric acid equals about 10 MBTU/lb (730 KCAL/mol of P.sub.4) and can be utilized in the evaporation of water from dilute phosphate solutions or phosphoric acid solutions that require concentration before subsequent processing.
In recent years, the oxidation of phosphorus to phosphorus pentoxide has taken place in stainless steel water jacketed chambers wherein the heat of combustion is dissipated by conduction to the circulating water. With the advent of these improved combustion chambers, there has been a concomitant interest in devising methods for the recovery of the waste heat values generated during the conversion of phosphorus to phosphorus pentoxide.
It has been suggested to recover low level (&lt;100.degree. C.) heat from wet manufacturing processes by operating on the Rankine cycle. This process, however, would only recover the low level heat which amounts to &lt;5 percent of the heat values produced.
Japanese Pat. No. 84,890, reported in Derwent Publication No. 60687B (1979), is said to disclose a phosphoric acid plant wherein corrosion is prohibited by predrying the combustion air to prevent the formation of corrosive polyphosphoric acids and heat transfer surfaces are operated above the dew point of P.sub.2 O.sub.5. The disclosed process requires that the boiler tubes be directly exposed to the combustion gases, thereby creating the possibility of uneven heat transfer.
An article in Chemical Week, Jan. 31, 1979 issue, page 36, disclosed that Battelle Laboratories was utilizing Battelle's multisolid fluidized-bed combustion process to burn solids and sewage sludges. The unit is described as burning waste mixed with sand while the mixture is suspended in a vertical air stream. The hot sand is then separated and recirculated through an external boiler where it is utilized to dry the sludge and vaporize water to produce steam.
Another article in Chemical Week, Oct. 17, 1984, page 39, discloses that second generation fluidized-bed combustion chambers for coal can sidestep the problem of erosion and corrosion of steam-generating tubes in the fluidized bed by removing the heat-recovery zone to other places.
U.S. Pat. No. 4,450,146 discloses a process for the recovery of heat values from the oxidation of phosphorus to phosphorus pentoxide wherein the phosphorus is oxidized to P.sub.2 O.sub.5 in a fluidized bed and the fluidized-bed particles are then contacted with water to produce steam. While a somewhat viable alternative, the disclosed process suffers from the disadvantage of requiring the energy-inefficient removal of the fluid bed material, and then contacting the removed material with water to produce steam. The particles must also be dried before being returned to the bed.